Support AC::Parameters for PG HStore

As reported via #26904, there is a regression in how values for
Postgres' HStore column type are being processed, beginning in Rails 5.
Currently, the way that Active Record checks whether or not values need
to be serialized and put into the correct storage format is whether or
not it is a `Hash` object. Since `ActionController::Parameters` no
longer inherits from `Hash` in Rails 5, this conditional now returns
false. To remedy this, we are now checking to see whether the `value`
parameters being passed in responds to a certain method, and then
calling the `serialize` method, except this time with a real Hash
object. Keeping things DRY!

Fixes #26904.

activerecord/CHANGELOG.md

+*   Allow `ActionController::Parameters`-like objects to be passed as
+    values for Postgres HStore columns.
+
+    Fixes #26904.
+
+    *Jon Moss*
+
 *   Added `stat` method to `ActiveRecord::ConnectionAdapters::ConnectionPool`
 
     Example:

activerecord/lib/active_record/connection_adapters/postgresql/oid/hstore.rb

@@ -24,6 +24,8 @@ def deserialize(value)
           def serialize(value)
             if value.is_a?(::Hash)
               value.map { |k, v| "#{escape_hstore(k)}=>#{escape_hstore(v)}" }.join(", ")
+            elsif value.respond_to?(:to_unsafe_h)
+              serialize(value.to_unsafe_h)
             else
               value
             end

activerecord/test/cases/adapters/postgresql/hstore_test.rb

@@ -10,6 +10,12 @@ class Hstore < ActiveRecord::Base
       store_accessor :settings, :language, :timezone
     end
 
+    class FakeParameters
+      def to_unsafe_h
+        { "hi" => "hi" }
+      end
+    end
+
     def setup
       @connection = ActiveRecord::Base.connection
 
@@ -321,6 +327,10 @@ def test_schema_dump_with_shorthand
       assert_match %r[t\.hstore "tags",\s+default: {}], output
     end
 
+    def test_supports_to_unsafe_h_values
+      assert_equal("\"hi\"=>\"hi\"", @type.serialize(FakeParameters.new))
+    end
+
     private
       def assert_array_cycle(array)
         # test creation